Scanning Probe Microscopes (SPMs) are instruments that provide high resolution information about the properties of surfaces. Scanning Probe Microscopes are commonly used for imaging, with some SPMs being able to image individual atoms. Along with images, SPMs can be used to measure a variety of surface properties, with detail over the range from a few angstroms to hundreds of microns. For many applications, SPMs can provide both lateral and vertical resolution that is not generally obtainable from any other type of device.
One type of SPM is the atomic force microscope (AFM), which scans a sharp tip across a surface. The tip is mounted on the free end of a cantilever (lever). The tip is brought to a surface and the force interaction of the tip with the surface causes the cantilever to deflect. The deflection of the cantilever is measured and the position of the tip or sample can be used to adjust the vertical position of the tip as it is scanned so that the deflection, and thus the force, is kept substantially constant. The tip vertical position versus horizontal scan provides the topographic surface map. In AFM, the tip-sample interaction forces can be made very small, so small as not to deform biological molecules. Atomic force microscopes can also be operated in a non-contact mode where the repulsive force deflects the cantilever as it scans the surface. The deflection of the tip as it is scanned provides topographic information about the surface.
Atomic force microscopes are able to detect the small movements of the cantilever. Several techniques for cantilever motion detection have been used with the most common method employing reflected light from the cantilever. The deflection of a light beam due to the cantilever motion may be detected, or the movement of the cantilever can be used to generate interference effects which can be used to derive the motion. Atomic force microscopes can be used to image individual atoms as well as for measuring mechanical properties of the sample such as stiffness.
Probing devices have been developed for measuring such properties as electric field, magnetic field, photon excitation, capacitance, and ionic conductance. Whatever the probing mechanism, most SPMs have common characteristics, typically operating on an interaction between probe and surface that is confined to a very small lateral area and is extremely sensitive to vertical position. Most SPMs are able to position a probe very accurately in three dimensions and use high performance feedback systems to control the motion of the probe relative to the surface.
The positioning and scanning of the probe is usually accomplished with piezoelectric elements. These devices expand or contract when a voltage is applied to them and typically have sensitivities of a few angstroms to hundreds of angstroms per volt. Scanning is implemented in a variety of ways. Some SPMs hold the probe fixed and attach the sample to the scanning mechanism while others scan the probe. Piezoelectric tubes are commonly used, and are generally capable of generating three dimensional scans. They are mechanically stiff; have good frequency response for fast scans, and are relatively inexpensive to manufacture and assemble.
FIG. 1 is a simplified block diagram of an exemplary AFM 10. The AFM 10 includes, among other components, an actuator assembly, XYZ actuator assembly or scanner 12, and a controller or control station 14. Control station 14 typically consists of at least one computer and associated electronics and software that perform the tasks of data acquisition and control of the AFM. The control station 14 may consist of a single integrated unit, or may consist of a distributed array of electronics and software. The control station may use a typical desktop computer, a laptop computer, an industrial computer and/or one or more embedded processors.
The scanner 12 is mounted over a sample 16 in this case and bears a probe 18 on its lower, moving end. Probe 18 has a cantilever 20 and a probe tip 22 mounted on the free end portion of the cantilever 20. Again, in some cases, the probe tip 22 is positioned by the piezoelectric scanner 12 over a stationary sample 16; or, in some cases, the sample 16 is attached to the scanner 12 and the tip 22 is stationary. The probe 18 is coupled to an oscillating actuator or drive 24 that is used to drive probe 18 to oscillate at or near the probe's resonant frequency. Commonly, an electronic signal is applied from an AC signal source 26 under control of the AFM control station 14 to the drive the AC signal source 26 to oscillate probe 18, such as at a free oscillation amplitude Ao. The control station 14 acquires data from the sensing device 28 and through feedback controls the height of the tip 22 by applying control voltages to the scanner 12. The sensing device or detector 28 senses tip deflection. The x and y positions are controlled by applying voltages to the scanner through x and y drivers. Typically for most applications, a raster scan is generated by producing a linear motion in the x and y scan directions. The scan area can be offset by starting the raster from a selected position within the scanner range. The probe tip 22 in this arrangement can be positioned anywhere in x and y within the range of the scanner.
In operation, as the probe 18 is oscillated and brought into contact with sample 16, sample characteristics can be monitored by detecting changes in the oscillation of probe 18. In particular, a beam of light is directed towards the backside of probe 18 which is then reflected towards detector 28, such as a four quadrant photodetector. As the beam translates across the detector, appropriate signals are transmitted to control station 14 which processes the signals to determine changes in the oscillation of probe 18. Control station 14 generates control signals to maintain a substantially constant force between the tip 22 and the sample, typically to maintain a setpoint characteristic of the oscillation of probe 18. For example, control station 14 is often used to maintain the oscillation amplitude at a setpoint value to insure a generally constant force between the tip 22 and the sample 16. In other cases, a setpoint phase or frequency is used. The data collected by the control station 14 is typically provided to a workstation that manipulates the data obtained during scanning to perform the point selection, curve fitting, distance determining operations, and other functions. For some AFMs, the workstation is the control station. For other AFMs, the workstation is a separate on-board controller, a separate off-board controller, or any combination of the three.
In existing microscopes, drift of the probe tip across the sample is a significant effect. The drift can distort the image and can make it difficult to continue imaging the same feature over time. Typically, drift in the x-y plane is several angstroms per minute after the set-up has stabilized. Drift can be much greater when a sample is first contacted, sometimes requiring several hours of stabilization before accurate scanning can occur. Drift is due to thermal expansion of the piezoelectric scanner as well as the sample itself and its holder. Additional drift contributions due to “creep” and hysteresis of the piezoelectric material are often present but drift due to thermal expansion is typically the most pronounced. Some drift, such as the drift due to thermal effects, are long-term and are typically constant over the scanning of single images. Drift present when imaging with a scanning probe microscope can restrict its ability to dwell on atomic dimension features, which is useful for monitoring local processes or acquiring repeated images of unique structures. Drift also can cause inordinately long stabilization times before undistorted images can be acquired for larger images.
Many existing designs attempt to reduce the drift by controlling the position of the probe during the scanning process. Other attempts to reduce drift include matching thermal coefficients for probe materials or using superstructure materials that are primarily susceptible to thermal effects such as drift and thus are very stable. For instance, Invar®, a steel alloy, is a registered trademark of Imphy Alloys of Puteaux, France. Invar is commonly used in the construction of AFMs to minimize thermal drift. While Invar will become heated during a scan, the Invar steel alloy will not expand and therefore not drift.
More specifically, Invar is a nickel steel alloy that has a low coefficient of thermal expansion. As a result, it is commonly used in the construction of scientific instruments. While Invar has certain characteristics, such as a low coefficient of thermal expansion, that make it well suited for AFMs, Invar is not widely available and thus is costly. This cost can make purchase and use of instruments such as AFMs impracticable. A cost effective, low drift AFM solution was desired.